There is some debate on whether spacetime itself existed prior to the introduction of energy. If spacetime did not exist prior to the introduction of energy, then all coordinates and energy in this model would have had a zero value. If however, spacetime existed first, then all spatial coordinates would have been some positive number, and the time coordinate would have been negative value that counted up to zero at which time energy was introduced and the Big Bang happened.
The Big Bang Theory does not attempt to resolve the question of what larger construct all the required coordinates must have existed within to begin with. The result is a paradox whereby a dimensionless coordinate ( single point ), should not be capable of originating anything at all. To get around this paradox, the rules of logic tend to get bent. For example, it might be said that while everything originated from a point, it was not nothing, it was a point of infinitesimally small size. These types of explanations provide no meaningful insight other than demonstrating that those who pretend to know what happened at this early stage really don't have the foggiest idea.
After the introduction of energy into spacetime, the Big Bang Theory suggests that the universe expanded rapidly in all directions and over billions of years, condensed into matter which formed stars, planets and galaxies. There is a wealth of circumstantial evidence to support this hypothesis, the most convincing of which is that most galaxies appear to be moving away from one another, implying a common starting point in the distant past. Also, the discovery of the Cosmic Background Radiation in 1964 by Arno Penzias and Robert Wilson suggests that a smaller and denser volume of energy has been expanding in all directions for billions of years.